1. Field of the Invention
The present invention relates to a guided-wave optical branching device preferably used for a component in an optical communication system and, in particular, to a guided-wave optical branching device in which the wavelength dependence characteristics of a power coupling ratio is reduced.
2. Description of the Prior Art
In order to promote a commercial communication service using optical fiber transmission technologies, it is required to develop various kinds of optical devices in addition to optical fibers, light emitting devices and light receiving devices. Among these optical devices, an optical branching device is one of the most essential optical components, and it is required to provide optical branching devices with various values of a branching ratio or a coupling ratio such as 50%, 20% or several %. Especially, there is a great demand for an optical branching device having a coupling ratio which is less dependent on wavelength in a wide range of wavelength.
Optical branching devices are also called optical couplers, and based on their device configuration, are classified into (1) bulk type, (2) fiber type and (3) waveguide type.
A bulk type device is fabricated by combining micro optical lenses, prisms, interference filters with interference films and so on with each other to realize an optical branching device with a lower degree of wavelength dependence. The bulk type device is now in practical use. However, some problems remain unsolved in the bulk type optical branching device with respect to long fabricating and adjusting time, long-term reliability, cost and device size.
A fiber type device uses an optical fiber material and is fabricated by the steps of abrading, drawing and fusion splicing. It is possible to form an optical branching device with a lower degree of wavelength dependence. However, there is a disadvantage in that a skilled craftsman's work is required in the manufacturing process and reproducibility of the device is poor, so that this device is not suitable for mass-production.
In contrast to the above two types of optical branching devices, the guided-wave type device has an advantage in that a large number of devices can be fabricated at the same time or mass-manufactured on a single planar substrate by using a photolithography process and is a noteworthy optical branching device to be used widely in the future.
FIGS. 36A-36C show a plan view and cross sectional views of a structure of a conventional symmetrical optical branching device. FIG. 37 illustrates its characteristics with respect to its coupling ratio. This first type of prior art optical branching device is disclosed in "Silica-Based Single-Mode Waveguides on Silicon and their Application to Guided-Wave Optical Interferometers" by Norio Takato et al., JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 6, No. 6, June 1988, pp 1003-1010.
In the optical branching device as shown in FIGS. 36A-36C, two optical waveguides 21a and 21b, each having the same width, are placed on a planar substrate 21, and a directional coupling portion 22 is formed by making portions 22a and 22b of the optical waveguides 21a and 21b in close proximity to each other. Components 23a and 23b are input ports of the optical waveguides 21a and 21b, respectively, and components 24a and 24b are output ports of the optical waveguides 21a and 21b, respectively.
It should be noted that in the present specification, an optical waveguide means not only a combined structure of a core and a clad but also a core itself. In FIGS. 36B and 36C, cores as the optical waveguides 21a, 21b, 22a and 22b are buried in a clad layer 25 arranged on the substrate 21.
The coupling ratio as a function of wavelengths for the first prior art optical branching device, where the width of the two optical waveguides 22a and 22b is the same within the entire coupling portion 22, is a substantially sinusoidal function as shown in FIG. 37. For example, even if the coupling ratio is 50% at a wavelength of 1.3 .mu.m, the coupling ratio is increased to 100% at a wavelength of 1.55 .mu.m. This means that the coupling ratio varies by a relatively large amount even if the wavelength fluctuates by a small amount, so that the coupling ratio characteristics have a substantial wavelength dependence.
FIGS. 38 and 39 are a plan view showing a structure of an asymmetrical optical directional coupler and its characteristics with respect to its coupling ratio. This structure is disclosed, for example, in Laid-Open Japanese Patent Application No. 287408/1990.
In the optical directional coupler shown in FIG. 38, two optical waveguides 31a and 31b having constant widths which are different from each other are placed on a planar substrate 31, and a directional coupling portion 32 is formed by making portions 32a and 32b of the optical waveguides 31a and 31b in close proximity to each other. The directional coupler 32 is so designed that an input light wave from an input port 33a is branched to output ports 34a and 34b. Tapered waveguide portions 35a and 36a are connected to portions of the waveguide 31a in the vicinity of the input and output ports 33a and 34a so that the tapered waveguide portions 35a and 36a have a smoothly tapered shape so as not to produce a radiation mode in order to establish a good matching with external optical fibers to be connected to the input and output ports.
In the second example of a prior art device shown in FIG. 38, in order to realize a branching operation at a specific coupling ratio in a wide wavelength range, the width of the specific portion 32a and that of the specific portion 32b of the optical waveguides 31a and 31b, respectively, in the coupling portion 32 are made different from each other, while each width has a constant value. The device thus structured can be designed so as to establish a coupling ratio such as 50%.+-.10%, 20%.+-.5% and 5%.+-.3% in a desired wavelength range, as shown in FIG. 39.
However, in the case of designing an optical coupler which should satisfy a severe condition, for example, where the coupling ratio is 50%.+-.1% in the vicinity of a wavelength of 1.4 .mu.m and the coupling ratio is 50%.+-.2% in the vicinity of the wavelength region between 1.3 .mu.m and 1.5 .mu.m, there is a disadvantage in that a wavelength range (1.3 .mu.m-1.5 .mu.m) having a flat coupling ratio characteristic is varied and a designed coupling ratio is not attained in the desired wavelength range between 1.3 .mu.m and 1.5 .mu.m. And also, as seen in FIG. 38, the coupling ratio characteristic in the desired range of the wavelength is not flat but varies between several % and ten plus several %, and therefore, in the second example of the prior art, a guided-wave optical branching device of a wide wavelength operation type is not satisfactorily formed, and accordingly cannot be applied to an optical communication system.
Furthermore, in the device of the second example of the prior art, the radiating power loss in the curved portion of the narrow optical waveguide is relatively high. For example, the power loss is increased by about 0.7 dB more than for a conventional symmetrical directional coupler of the first example of the prior art having a constant waveguide width. Thus, the device is not applicable to an optical communication system. This is because the widths of the input and output ports are not the same in the entire directional coupler of the second prior art example.